The present invention refers to a valve device according to the precharacterizing portion of claim 1.
It is known to use such valve devices in connection with a combustion plant comprising a pressurized fluidized bed, a so called PFBC-plant (Pressurized Fluidized Bed Combustion). Such plants may comprise a gas turbine for extracting energy from the pressurized, hot combustion gases produced in the bed, and the compressed combustion air required at the combustion may be supplied to the bed via an air channel which has an annular cross-section and encloses the hot gas channel conveying the combustion gases from a combustion chamber to the gas turbine. Furthermore, it is known to provide such channels with a valve device which in the case of a gas turbine trip, i.e. that the gas turbine has to be stopped quickly, isolates the gas turbine from the pressure vessel.
SE-B-456 757 discloses such a valve device which is intended to be used in a PFBC-plant. The known valve device comprises two throttle valves which are provided one after the other in the hot gas channel and which are rotatable about a respective rotational axis. Furthermore, it comprises four cone valves which are distributed around the periphery of the air channel and which are intended to intercept the annular air channel and concurrently open a connection between the annular air channel and the hot gas channel. These four cone valves are provided in parallel, i.e. all four of them are to function in order to intercept the flow through the air channel. Although this known valve device has functioned satisfactorily in the plants where it is mounted today, it is connected with certain problems appearing in particular by the construction of very large combustion plants. Furthermore, these cone valves require powerful adjustment members for their operation and a valve spindle guiding which is provided in direct connection to the hot gas, which results in the problem of jamming of the valve. Another problem connected to this known valve device is that it comprises a large and complicated casting. Such castings are not possible to manufacture at least not to justifiable costs in the case of large plants having great flows. In addition, the known valve device comprises a fairly complicated flow path which results in high pressure drops at least with respect to the air channel.
EP-A-721 062 discloses such a valve device intended for a PFBC-plant. In this case, the whole valve device including all three valve functions is arranged in a common valve housing having significant dimensions. As appears from this document, the gas turbine and the compressor are provided above said valve housing. Furthermore, all three valve functions are depending on one adjustment member and provided on a common shaft. From a security point of view, this known valve device thus has a very small redundancy.